This invention relates to a digital system that is used to detect low peak power Low Probability of Intercept (LPI) signals and to suppress strong conventional pulsed signals, and more particularly, to such a system which utilizes an LPI Discriminator.
Various reconnaissance systems are used to intercept radar signals and decipher some of their critical characteristics and angles of arrival. A microwave intercept receiver may be used for just this purpose. In particular reconnaissance applications in areas such as Electronic Warfare (EW) the receiver is designed to fulfill roles such as radar warning, electronic support measures (ESM), and Electronic Intelligence (ELINT). In most conventional approaches, the intercept receiver is designed to perform two functions The first function is to measure the signal characteristics of the intercepted signal, and the second is to determine its angle of arrival (AOA) for the purpose of direction finding (DF) and location of the radar source.
With the proliferation of radar systems and the increasing number of radars employing complex waveform modulation, it is difficult to differentiate and sort the intercepted radar signals using just the coarse conventional parameters. Typically these coarse parameters include AOA, carrier frequency, pulse width (PW), pulse repetition interval (PRI), and scan pattern. Since many radars have similar conventional parameters, ambiguity may occur in both the sorting and identification processes.
One type of receiver that may be used to precisely measure the conventional parameters as well as the intrapulse modulation for both sorting and identification purposes is the intrapulse receiver.
However, the use of Low Probability of Intercept (LPI) radars with low peak power has introduced a further requirement for modem intercept receivers, requiring them to have a much higher sensitivity in order to detect these LPI radar signals. Until recently, almost all radars were designed to transmit short duration pulses with a high peak power. This type of signal is easy to detect using relatively simple, traditional EW intercept receivers making the attacker (radar source) vulnerable to either antiradiation missiles or Electronic Counter Measures (ECM). However, by using LPI techniques it is possible to design a LPI radar that is effective against traditional EW intercept receivers. One of the most important LPI techniques is the use of phase or frequency waveform coding to provide transmitting duty cycles approaching one. This technique can result in drastic reductions in peak transmitted power while maintaining the required average power.
Therefore with an increasing number of radars employing complex waveform modulation in addition to using low-peak power LPI signals, it is required that a modern intercept receiver perform the following three basic functions: a) measure and characterize conventional pulsed radar signals; b) detect and characterize LPI signals; and c) determine the AOA for both conventional pulsed signals and LPI signals. Furthermore, these three functions should be performed on the intercepted signals in a multiple signal environment and on a pulse-by pulse basis.
A current architecture that accomplishes both signal measurement and accurate AOA determination on conventional pulsed signals is an interferometer. In an interferometer, a number of antenna elements are distributed in a two-dimensional plane and phase comparison between different antenna elements is used to determine the AOA. Microwave phase detectors are typically used for phase comparison. Recently these phase detectors have been replaced by digital measurement techniques. The signal characteristics of the intercepted signals are measured either from the output of one of the interferometer antennas or from a separate antenna. Signal characterization is performed using an intrapulse receiver implemented by analog devices. In this case, a frequency discriminator is used for frequency measurement while a Detector Log Video Amplifier (DLVA) is used for amplitude measurement.
Detection of LPI signals is currently accomplished using a channelized receiver instead of an intrapulse receiver. A channelized receiver is typically implemented using either a band of microwave filters with a detector at the output of each filter. Other receivers may be used, such as a time-integrating acousto-optic spectrum analyzer and compressive receiver. The use of a channelizer will reduce the noise bandwidth in each channel and thus increase the receiver sensitivity for LPI signal detection. Other architectures such as correlators are also suitable for LPI signal detection and AOA determination. These correlators are implemented using analog, optical, or digital technology. However, the AOA determination process is quite different from the interferometer approach and very limited intrapulse information can be extracted.
As mentioned above, digital signal processing technology is used in both the single-channel and the multi-channel receiver architectures. The potential advantages of digital receivers are robustness, flexibility and cost. However, the processing functions can be quite complex and numerous once the IF signals digitized. This is especially true in the case of the multi-channel digital receiver architecture. It is still quite a challenge for the digital signal processing technology to meet all of the processing requirements if all of the digitized data from the Analog-to-Digital Converters (ADCs) are to be processed in or near real-time.
It is to be noted that the vast majority of radar signals are of pulsed nature and the duty cycles are relative low. Therefore, the portion of the digitized data set where the signals are actually present could be quite small in a typical signal environment. Most conventional radars transmit high-peak power and pulses with pulse width less than 1 xcexcs. However, LPI signals are generally characterized by very low-peak power and are of much longer duration. Pulse widths of LPI signals are expected to be 5 xcexcs and longer. Digitized IF signals of an LPI signal and a pulsed signal are shown in FIG. 1. If a threshold is used on the digitized data to reduce the amount of data to be passed on to the processor, the LPI signal will likely be missed and undetected.
It is an object of the present invention to obviate or mitigate the above disadvantages.
The present invention seeks to provide a solution to the problem of radar processor overload when LPI signals are present with conventional pulsed signals.
An advantage of the present invention is to enhance the detection of weak LPI signals.
A further advantage of the present invention is to suppress the presence of high-peak power and short-duration conventional pulsed signals, and to trigger a data buffer for gating digitized LPI data to a processor for processing.
In accordance with this invention there is provided an LPI signal discriminator comprising an amplifier for receiving an incoming IF signal; a signal detector; and a comparator responsive to the output of the signal detector for producing a trigger signal when the detected signal is above a predetermined threshold.
A still further embodiment of the invention provides for a digital receiver for determining parameters of an incoming signal, comprising: one or more receiver channels, each channel including a respective antenna for receiving the incoming signal; a down converter for converting the signal to an intermediate frequency (IF) signal; an analog-to-digital converter (ADC) operatively coupled to receive the IF signal and to provide a digital signal output in response to a trigger signal, the digital signal being indicative of the phase and amplitude of the received signal in the channel; a digital processor operatively coupled to receiving the digital signals from each of the plurality of channels and for determining the parameters by utilizing the phase and amplitude on a pulse by pulse basis; and an LPI signal discriminator operatively coupled to the down converter for producing the trigger signal when the incoming signal is above a predetermined threshold, to thereby transfer the digitized signal to the signal processor for processing the incoming LPI signal.